Exploring the Limits of the Nernst Equation in Concentration Cell Experiments

[somasimple]

Hi All,
(Just for information and understanding),
If we have a concentration cell experiment like the image uploaded, in a concentration cell, the solutes are supposed to be of the same kind, iI.e. CuCl2 (?).
I suppose it is possible to add to the left vat a m quantity or volume of CuSO4 to increase the RedOx reactions ?
In my experiment, the vats have the same volumes.

How the Nernst equation is modified in that case ?

 

[Borek]

in a concentration cell, the solutes are supposed to be of the same kind, iI.e. CuCl2 (?).

No, the only thing that matters is that the reacting ion is present on both sides, counterions can be completely random (that is, as long as they don't interfere).

How the Nernst equation is modified in that case ?

Trivial to derive - just ratio of concentrations.

 

[somasimple]

Thanks,
So the reacting ion present on both sides is Cu++.

Trivial to derive - just ratio of concentrations.

For a Chemist for sure!
for example we have 2/3 volume of CuCl2 100 mMol and 1/3 CuSO4 50 mMol at left
and
100% CuCl2 4 mMol at right?

 

[Borek]

No, you are overcomplicating things. All you need is to calculate the final concentration of Cu²⁺ (from all sources) in each cell.

http://www.chembuddy.com/?left=concentration&right=dilution-mixing

 

[somasimple]

Thanks for the reply.
Does that means if we do not have access/know the volumes of each concentration, the computation is not possible?

 

[Borek]

Does that means if we do not have access/know the volumes of each concentration, the computation is not possible?

Yes, that's what it means.

 

[somasimple]

Thanks,
A last (?) question: What happens when the cell volumes are different, V1 for the left and V2 for the right one ?

 

[Borek]

Volumes don't matter when it comes to the potential, Nernst equation contains only concentrations.

Amounts of substances play a role when you want to find the capacity of the battery, but that's another story.

 

[somasimple]

Thanks for the reply.
it is said the Nernst equation becomes useless when concentrations are too low or too high.
http://www.chem1.com/acad/webtext/elchem/ec-4.html
What are the upper and lower limits of its use?

 

[Borek]

Lower limit - listed in the article you linked to.

Upper limit - depends on what you mean by "equation becomes useless". There are ways of extending its use (they are based on a concept of ionic strength, activity and activity coefficients, listed in the text), and if memory serves me well these extended approaches (like Specific Interaction Theory) work OK to concentrations in the 4-5 M range (which is around a solubility limit for most salts). So the answer is either "works perfectly up to 0.01 M", or "there is no upper limit ".